The present invention relates to a thermoacoustic engine for converting acoustic energy to thermal energy or for converting thermal energy to acoustic energy. More particularly, the present invention relates to a thermoacoustic engine with an acoustically isolated heat exchanger.
Thermoacoustic engines have developed as an attractive alternative to more traditional piston and turbine devices for heating, cooling, and electric power generation applications. Thermoacoustic engines are generally highly reliable due to the limited number of moving parts and the abrogated need for lubrication. Furthermore, thermoacoustic systems are environmentally friendly as they can utilize air or a noble gas as a heat transfer medium and working fluid rather than poisonous or ozone layer damaging substances, such as FREON, which are commonly used in conventional piston and turbine devices. In what follows the terms heat transfer medium and working fluid will be used interchangeably for brevity, unless otherwise indicated.
FIG. 1 illustrates a typical thermoacoustic engine 10 including an acoustic resonator 12 and a drive tube 14. Drive tube 14 is a hollow, elongated member typically having a closed end 16 and an open end 18. Open end 18 is connected or sealed to acoustic resonator 12. Drive tube 14 contains a first thermal element 20, a regenerator 24, and a second thermal element 22. As illustrated, first thermal element 20 is positioned further from acoustic resonator 12 than second thermal element 22, and regenerator 24 is positioned between first thermal element 20 and second thermal element 22. First thermal element 20 is commonly a heat source and second thermal element 22 is commonly a heat sink. Acoustic resonator 12 and drive tube 14 are generally filled with a heat transfer medium 26, which is typically air or a noble gas. Heat transfer medium 26 flows through and between first thermal element 20, regenerator 24, and second thermal element 22 to facilitate thermal exchange.
During operation, heat is supplied to first thermal element 20 while heat is simultaneously removed from second thermal element 22 to establish a sufficient temperature gradient across regenerator 24 to activate thermoacoustic engine 10. Upon activation, thermoacoustic engine 10 may function as a Carnot engine in which first thermal element 20 is heated to induce movement in heat transfer medium 26 to produce a high intensity sound in acoustic resonator 12. Alternatively, acoustic energy is introduced to heat transfer medium 26 which is employed to establish thermal transition from the cold sink, i.e., second thermal element 22, across regenerator 24, to the heat source, i.e., first thermal element 20, to function as a refrigerator.
Typical thermoacoustic engines, such as thermoacoustic engine 10, depend on thermal conduction through the drive tube walls at first and second thermal elements 20 and 22. In particular, heat exchangers or electric elements are commonly attached to the inside or outside of the drive tube 14, such as at the first and/or second thermal elements 20, 22 located within drive tube 14, to add or remove heat from the respective elements.
The typical thermoacoustic engines have low thermal efficiency, low power density, and tend to be significantly larger than their piston or turbine driven counterparts. A significant factor contributing to the aforementioned disadvantages of thermoacoustic engines is a difficulty in supplying or removing heat to or from the active areas or thermal elements of the thermoacoustic engine while maintaining acceptable acoustic losses.
To avoid the shortcomings of the above-discussed thermoacoustic engines and for other reasons presented in the Description of the Preferred Embodiments, a need exists for a thermoacoustic engine which supplies and removes heat from the respective portions of the drive tube in a more efficient manner so as to maintain acceptable levels of acoustic losses.
One aspect of the present invention provides a thermoacoustic engine for acoustically driving a thermal exchange. The thermoacoustic engine includes a hollow drive tube, a heat transfer medium, an acoustic resonator, and a first thermal element. The hollow drive tube partially contains the heat transfer medium and is connected to and opens into the acoustic resonator. The acoustic resonator is adapted to store acoustic energy and deliver at least one acoustic wave to the heat transfer medium. The first thermal element includes a first channel and a first working fluid. The first channel is positioned to cross and open into the hollow drive tube, at least partially contains the first working fluid, and is sized to decrease the propagation of the at least one acoustic wave within the first channel. The first thermal working fluid is adapted to interact with and undergo thermal exchange with the heat transfer medium by conduction.
In one embodiment, the first channel is sized to procure exponential decay of the acoustic waves within the first channel. Additionally, the first channel has a duct-cut off frequency smaller than a frequency of the hollow drive tube (i.e. a critical dimension smaller than a dimension required for propagation of the at least one acoustic wave). In one embodiment, the first thermal element further includes an external heat exchanger connected and open to a first end and a second end of the first channel. The heat exchanger is adapted to alter the thermal energy of the first working fluid.
In another embodiment, the thermoacoustic engine further includes a second thermal element spaced from the first thermal element. The second thermal element includes a second channel at least partially containing a second working fluid. The second channel is positioned to cross and open into the hollow drive tube and is sized to decrease propagation of the at least one acoustic wave within the second channel. The second working fluid is adapted to interact and undergo thermal exchange within the heat transfer medium.
Another aspect of the present invention provides a thermoacoustic engine for producing at least one acoustic wave. The thermoacoustic engine includes a drive tube, an acoustic resonator, a heat transfer medium, a first thermal element, and a second thermal element. The drive tube is connected to and opens into the acoustic resonator, and the drive tube and acoustic resonator contain the heat transfer medium. The first thermal element includes a first channel positioned to cross and opens into the drive tube. The first working fluid is at least partially contained in the first channel and is adapted to interact and undergo thermal exchange with the heat transfer medium by conduction. The second thermal element is spaced from the first thermal element and is adapted to induce thermal exchange between the second working fluid and the heat transfer medium. Thermal exchange between the first thermal element and the heat transfer medium and between the second thermal element and the heat transfer medium produces an acoustic wave in the heat transfer medium. The first channel is sized to decrease propagation of the acoustic wave within the first channel.
Another aspect of the present invention provides a method of acoustical thermal exchange. The acoustical method includes providing a thermoacoustic engine, inducing an acoustic wave, and exchanging thermal energy. The thermoacoustic engine provided includes a drive tube, a heat transfer medium contained in the drive tube, a first channel, and a first working fluid at least partially contained in the first channel. The first channel is positioned to cross and open into the drive tube. Introducing an acoustic wave to the drive tube induces flow within the heat transfer medium. The first channel is sized to decrease propagation of the acoustic wave within the first channel. Exchanging thermal energy occurs between the heat transfer medium and the first working fluid by conduction.